In recent years, battery systems incorporating a large number of batteries, such as power storage devices for moving objects, power storage devices for stabilizing grid interconnection, and emergency power storage devices, have attracted attention. To derive performance of these systems, parameters such as a battery charge rate (hereinafter referred to as SOC), a deterioration rate (SOH), a maximum chargeable/dischargeable current (allowable current value) are calculated and used for battery control, or it is necessary to properly equalize the charge rate of each battery.
To realize the above, a circuit (cell controller) for battery voltage measurement is attached to each battery, and based on information sent from these cell controllers, a battery controller mounting a central processing unit (CPU) performs the above-described calculation and operation. The calculation of the allowable current value is part of a safety function to prevent overvoltage of a battery, and safety of a battery system is maintained by limiting a current so as not to exceed the allowable current value.
To calculate the maximum current at which the battery does not become overvoltage, the internal state and parameters of the battery such as an open current voltage (hereinafter referred to as OCV) of the battery and internal resistance information. In particular, it is necessary to influence a polarization voltage generated in the battery in a power storage device for a moving object in which irregular current always flows. Considering calculation performance of the above-described CPU, it is particularly required to calculate a safe current in consideration of influence of an OCV, an internal resistance, and a polarization voltage in calculation of allowable current value for a moving object with a small calculation amount. However, to calculate the polarization voltage, it is necessary to use a function having a large calculation amount such as an exponential function and therefore difficult to calculate with the CPU.
Therefore, a method has been proposed in which a time during which a current continuously flows in a battery is measured, and a resistance value to be used for calculation of an allowable current value is referred to from a resistance value table reflecting influence of a polarization voltage by using the measured time (for example, refer to PTL 1). According to this, the allowable current value can be calculated with a small calculation amount without using an exponential function or the like. Further, as disclosed in PTL 1, a method is used in which a fixed resistance value corresponding to a sufficiently large polarization voltage is used for applications in which a current continuously flows for a short time as in a hybrid vehicle. Through these methods, it is possible to calculate, with a small calculation amount, a current which does not cause a battery to become overvoltage even when a polarization voltage exists.